Programmable gain amplifier with glitch minimization

ABSTRACT

A programable gain amplifier (PGA) has an amplifier and a variable resistor that is connected to the output of the amplifier. The variable resistor includes a resistor that is connected to a reference voltage and multiple parallel taps that tap off the resistor. A two-stage switch network having fine stage switches and coarse stage switches connects the resistor taps to an output node of the PGA. The taps and corresponding fine stage switches are arranged into two or more groups, where each group has n-number of fine stage switches and corresponding taps. One terminal of each fine stage switch is connected to the corresponding resistor tap, and the other terminal is connected to an output terminal for the corresponding group. The coarse stage switches select from among the groups of fine stage switches, and connect to the output of the PGA. During operation, one selected tap is connected to the output of the PGA by closing the appropriate fine stage switch and coarse stage switch, where the selected tap defines a selected group of the fine stage switches. Additionally, one fine stage switch is closed in each of the non-selected groups of fine stage switches. In one embodiment, the location of the closed switches in the non-selected groups is the mirror image of the location in an adjacent group. This reduces the transient voltages that occur when tap selection changes from one group to another.

[0001] This application claims the benefit of U.S. ProvisionalApplication No. 60/286,534, filed on Apr. 27, 2001, which isincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention generally relates to automatic gain controlin a receiver, and more specifically to a programmable gain amplifier(PGA) that performs automatic gain control while minimizing transientvoltages during tap changes.

[0004] 2. Background Art

[0005] In electronic communications, electromagnetic signals carryinformation between two nodes over a connecting medium. Exemplary mediainclude cable, optical fiber, public airways, etc. The signal strengthat the receiving node varies depending on the distance between the nodesand changes in the condition of the medium. For example, the signalstrength typically decreases with increasing distance between the twonodes. Furthermore, even if the distance is fixed, physical variationsin the medium over time can affect signal strength. For example, in acable system, different cables can have different attenuation constants.Also, increased moisture content in a cable line, or in the publicairways can reduce signal strength at the receiver. Finally, variationsin transmitter output power will also affect signal strength at thereceiver.

[0006] An automatic gain control (AGC) circuit and a programmable gainamplifier (PGA) are often used at the receiver input to compensate forvariations of received signal strength. More specifically, the AGCcircuit adjusts the gain setting of the PGA to maintain the signalstrength within a desired operating range. If the received signalstrength is too high, then the AGC lowers the gain setting of the PGA.If the received signal strength is too low, then the AGC raises the gainsetting of the PGA. When the AGC is changing the gain of the PGA, thereis a possibility of introducing a glitch in the system. The glitchmanifests itself as an unwanted transient voltage that can cause avoltage detection error if the transient voltage does not settle withinspecified time period, for example one clock cycle.

[0007] What is needed is PGA configuration that quickly settles anytransient voltage caused by changing gain settings. Furthermore, the PGAconfiguration should have sufficient operating bandwidth.

BRIEF SUMMARY OF THE INVENTION

[0008] The present invention is a programable gain amplifier (PGA)having an amplifier and a variable resistor that is connected to theoutput of the amplifier. The variable resistor includes a resistor thatis connected to a ground or reference voltage, and multiple paralleltaps that tap off the resistor. Additionally, the PGA includes atwo-stage switch network having fine stage switches and coarse stageswitches that connect the resistor taps to an output node of the PGA.The taps and corresponding fine stage switches are arranged into two ormore groups, where each group has n-fine stage switches andcorresponding taps. One terminal of each fine stage switch is connectedto the corresponding resistor tap, and the other terminal is connectedto an output terminal for the corresponding group. The coarse stageswitches are connected to corresponding group output terminals andselect a group of fine stage switches to connect to the output of thePGA.

[0009] During operation, one tap is selected to be connected to theoutput of the PGA by closing the appropriate fine stage switch andcoarse stage switch, where the selected tap defines a selected group ofthe fine stage switches. Additionally, one fine stage switch is closedin each of the non-selected groups of fine stage switches. In oneembodiment, the location of the closed switches in the non-selectedgroups is the mirror image of the location in an adjacent group. Inother words, if the m^(th) fine stage switch is closed in a first groupof fine stage switches, then the [(n+1)−m]^(th) fine stage switch isclosed a second group of fine stage switches that is adjacent to thefirst group of fine stage switches, assuming the fine stage switches areindexed from 1-to-n in each group. This reduces the transient voltagesthat occur when tap selection changes from one group to another.

[0010] Further features and advantages of the present invention, as wellas the structure and operation of various embodiments of the presentinvention, are described in detail below with reference to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

[0011] The present invention is described with reference to theaccompanying drawings. In the drawings, like reference numbers indicateidentical or functionally similar elements. Additionally, the left-mostdigit(s) of a reference number identifies the drawing in which thereference number first appears.

[0012]FIG. 1 illustrates an exemplary receiver environment having aprogramable gain amplifier (PGA);

[0013]FIG. 2 illustrates a conventional PGA 200;

[0014]FIG. 3A illustrates a PGA 300 with a two stage switchconfiguration according to embodiments of the invention;

[0015]FIG. 3B illustrates a parasitic capacitance associated with thePGA 300;

[0016] FIGS. 4A-4B illustrate example two stage switch PGAconfigurations with at least one switch turned on in each group of finestage switches;

[0017] FIGS. 5A-5E illustrate example two stage switch PGAconfigurations with one or more switches turned on in each group of finestage switches, according to embodiments of the present invention;

[0018]FIG. 6 illustrates the 3 dB cutoff frequency vs. PGA gain settingfor a PGA that is operated according to embodiments of the presentinvention; and

[0019]FIG. 7 illustrates a flowchart 700 of that describes the operatingthe switches in the PGA according to embodiments of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

[0020] 1. Example Receiver Application

[0021] Before describing the invention in detail, it is useful todescribe an example receiver environment for the invention. Theprogramable gain amplifier (PGA) invention is not limited to thereceiver environment that is described herein, as the PGA invention isapplicable to other receiver and non-receiver applications as will beunderstood to those skilled in the relevant arts based on thediscussions given herein.

[0022]FIG. 1 illustrates an environment 100 having a medium 102, and areceiver 106 that receives a communications signal 104 carried by themedium 102. The receiver 106 includes a programable gain amplifier (PGA)108, an analog-to-digital converter (ADC) 110, a digital signalprocessor (DSP) 112, and an automatic gain control (AGC) 116. Thereceiver 106 receives the communications signal 104 from the medium 102,and extracts an information signal 114. More specifically, the PGA 108receives the communications signal 104 and variable amplifies the signal104 as determined by the AGC 116 to generate a PGA output signal 109.The ADC 110 converts the PGA output 109 to a digital signal 111. The DSP112 processes the digital signal 111 to generate the information signal114. For example, the DSP 112 examines the voltage of the digital signal111 to determine if the voltage represents a “0” or a “1” in order toretrieve the information signal 114. The DSP 112 may also perform acyclic redundancy check (CRC) on the bit stream of the digital signal111 to determine if there have been any errors that were introducedduring transmission.

[0023] The signal strength of the input signal 104 can vary based on thephysical characteristics of the medium 102. In cable systems forexample, a longer cable will typically have more attenuation than ashorter cable, thereby affecting the signal strength of the signal 104.In order to compensate, the AGC 116 detects the signal strength of thedigital signal 111 and adjusts the gain settings of the PGA 108 usingAGC control signal 117 to maintain a relatively constant signalstrength. For example, if the signal strength of the digital signal 111is too weak, then the AGC 116 increases the gain setting of the PGA 108to increase the signal strength. Alternatively, if the signal strengthof the digital signal 111 is too strong, then the AGC 116 decreases thegain setting of the PGA 108 to decrease the signal strength.

[0024] Without AGC compensation, these signal strength variations wouldadversely affect the accuracy of the information signal 114. Forexample, if the received signal 104 is too strong, then the ADC 110 canbe saturated. Conversely, if the digital signal 111 is too weak, falsepositives can be generated during the CRC error check that is performedby the DSP 112.

[0025] 2. Conventional PGA

[0026]FIG. 2 illustrates a conventional PGA 200 that includes anamplifier 202 and a variable resistor 210 that is connected to theoutput of the amplifier 202. The amplifier 202 can be any type amplifierincluding a buffer amplifier. The variable resistor 210 includes aresistor 204 that connects the output of the amplifier 202 to ground ora reference voltage. The resistor 204 has multiple parallel taps 206 a-nthat tap off the resistor 204 (e.g. resistor ladder) to a common node214, which is the output of the PGA 200. Switches 208 a-n connect thecorresponding taps 206 a-n to the common node 214. The switches 208 arecontrolled by a control signal 212, such as the AGC 117.

[0027] During operation, the amplifier 202 amplifies the receivedcommunications signal 104 to generate an amplified signal 203. Theamplified signal 203 travels through the resistor 204, and is tapped offthe resistor 204 to the output 214 by a corresponding switch 208.Typically, only one switch 208 is closed at a time, so that only one tap206 is connected the common node 214. The tap 206 that is connected tothe common node 214 is referred to herein as the “selected tap”.

[0028] As such, the variable amplifier 210 provides a variable seriesresistance that attenuates the amplified signal 203, where theattenuation increases with increasing resistance. The resistance, andtherefore the attenuation, varies depending on which tap 206 isconnected the common node 214. The lowest resistance and attenuationoccur when the tap 206 a is the selected tap. The highest resistance andthe highest attenuation occur when the tap 206 n is the selected tap.The attenuation is increased by incrementally selecting taps in thedirection from 206 a to 206 n. Likewise, the attenuation is decreased byselecting taps in the direction of 206 n to 206 a.

[0029] For example, assume that switch 208 b is closed to select the tap206 b as an initial condition. The attenuation can be increased relativeto the initial condition by opening switch 208 b and closing switch 208c so as to select tap 206 c. The attenuation can be decreased relativeto the initial condition by opening the switch 208 b and closing theswitch 208 a to select the tap 204 a.

[0030] Typically, the PGA 200 is implemented on a integrated circuit(IC) where the circuit elements are deposited on the IC using knownlayout and processing techniques. Each switch 208 has a parasiticcapacitance to the IC ground, which causes an effective parasiticcapacitance 216 to ground at the common node 214, as shown in FIG. 2.The effective capacitance 216 limits the frequency bandwidth as will beunderstood by those skilled in the arts. Further, the effectivecapacitance 216 increases with the number of switches 208 (and thereforethe number of taps 206) because the switches 208 are in parallel, andparallel capacitance is cumulative. Therefore, the frequency bandwidthof the PGA 200 decreases as the number of taps 206 (and switches 208)increases. As a result, there is trade-off between the granularity ofthe attenuation (i.e. number of taps) in the PGA 200, and the frequencybandwidth of the PGA 200.

[0031] 3. PGA Description

[0032]FIG. 3A illustrates a PGA 300 according to one embodiment of thepresent invention. The PGA 300 includes the amplifier 202 and a variableresistor 301. Similar to the PGA 200, the variable resistor 301 includesa resistor 302 that connects the output of the amplifier 202 to groundor a reference voltage, and has multiple taps 304 that tap off theresistor 302 (e.g. resistor ladder). Additionally, the PGA 300 includesa two stage switch configuration that connects the taps 304 to an outputnode 310 of the PGA 300, instead of the single stage switchconfiguration in the PGA 200. More specifically, the taps 304 areconnected to the output node 310 by fine stage switches 306 and coarsestage switches 308. The taps 304 and corresponding fine stage switches306 are arranged into two or more groups 312, where each group 312 has agroup output terminal 307. One terminal of each fine stage switch 306 isconnected to the corresponding tap 304, and the other terminal isconnected to the group output terminal 307 for the corresponding group312. The output terminal 307 for each group 312 is connected to the PGAoutput node 310 by the corresponding coarse stage switch 308.

[0033] The nomenclature for the reference numbers in FIG. 3A is asfollows. The groups 312 of switches 306 have been indexed from 1-to-nmoving down the page. For example, the first group is 312-1, the secondgroup is 312-2, etc. The elements inside the groups 312 are given twoindex numbers after the “-” represented here as “-ab”. The “a”represents the specific group 312 number in which the elements arelocated, and the “b” represents the element index within the group 312.For example, all the switches 306 in group 312-1 are given acorresponding “-1” for the “a” index, and then numbered from 1-to-n forthe “b” index. As a result, the switches 306 in group 312-1 arereferenced as 306-11, 306-12, 306-13, . . . to 306-1n. The switches 306in group 312-2 are references as 306-21, 306-22, 306-23 . . . 306-2n. Aswill be apparent, there can be any number of switches 306 in aparticular group 312, and any number of groups 312. A greater number oftaps 304 permits smaller changes in incremental attenuation, as will beapparent to those skilled in the arts.

[0034] During operation, the amplifier 202 amplifies the receivedcommunications signal 104 to generate an amplified signal 203. Theamplified signal 203 travels through the resistor 302, and is tapped offthe resistor 302 at a selected tap 304 to the output node 310. Theamplified signal 203 is tapped off the resistor 302 by closing theappropriate switches 306 and 308. Therefore, the resistor 302 provides avariable series resistance that attenuates the amplified signal 203. Theamount of attenuation depends on which tap 304 is selected to beconnected to the output node 310 by the switches 306 and 308. A gaincontrol signal 303 determines the selected tap 304 by closing theappropriate fine stage switch 306 and coarse stage switch 308. Forexample, the gain control signal 303 can be an AGC signal, such as AGC117 in FIG. 1A.

[0035] Herein, the term “selected tap” will be used to refer to the tap304 that is connected to the output 310 by the switches 306 and 308.Similarly, the fine stage switch 306 that corresponds to the selectedtap 304 may be referred to as the “selected switch” 306. Similarly, thegroup 312 that contains the selected tap 304 and corresponding selectedswitch 306 may be referred to as the “selected group” 312.

[0036] One fine stage switch 306 and one coarse stage switch 308 areclosed in order to connect the selected tap 304 to the output node 310.For example, in order to select tap 304-11, then the fine stage switch306-11 and the coarse stage switch 308-1 are closed. In order to selecttap 304-23, the fine stage switch 306-23 and the coarse stage switch308-2 are closed. The lowest resistance, and therefore the lowestattenuation occurs when the tap 304-11 is the selected tap. The highestresistance, and therefore the highest attenuation, occurs when the tap304-nn is the selected tap. The attenuation is increased byincrementally selecting taps in the direction from 304-11 to 304-nn.Likewise, the attenuation is decreased by incrementally selecting tapsin the direction from 304-nn to 304-11. For example, if tap 304-12 isthe selected tap as an initial condition, then the attenuation can beincreased by changing the selected tap to tap 304-13. Likewise, theattenuation can be decreased by changing the selected tap to tap 304-11.

[0037] As in the conventional PGA 200, the switches 306 and 308 have aparasitic capacitance to ground that effects the frequency bandwidth ofthe PGA 300. The effective capacitance for each group 312 of switches306 is represented by capacitor 314 in FIG. 3B. The two stage switchconfiguration of the PGA 300 mitigates the effect of the groupcapacitances 314. This occurs because only the selected group 312 isconnected to the output node 310 by the corresponding (closed) switch308, and therefore only the parasitic capacitance 314 of the selectedgroup 312 is in the signal transmission path. The remaining non-selectedgroups 312 are isolated by the corresponding (open) switches 308. Forexample, if the tap 304 a is selected, then the switches 306 a and 308 aare closed. The remaining switches 308 are left open, and therefore onlythe effective parasitic capacitor 314 a of the group 312 a is connectedto the output 310. The remaining effective parasitic capacitors 314 areisolated from the output node 310 by their respective open switches 308.

[0038] The PGA 300 is illustrated as a singled-ended configuration.However, the PGA 300 can be configured as differential PGA, as will beunderstood by those skilled in the arts.

[0039] 4. Transient Voltage Considerations

[0040] Transient voltages can be created when the tap selection ischanged to vary the attenuation of the PGA 300. The transient voltageoccurs because the parasitic capacitances associated with switches 306and 308 store and release energy when the switches are closed andopened. For example, if the tap selection is changed from 304-1n (ingroup 312-1) to tap 304-21 (in group 312-2), then the switches 306-1nand 308-1 are opened, and the switches 306-21 and 308-2 are closed. Whenthe switch 306-1 n is opened, charge that was stored on the parasiticcapacitance of the switch 306-1n is discharged. Likewise, when theswitch 308-2 is closed, charge is transferred and stored on theparasitic capacitance of the switches 306-21 until the parasiticcapacitance is fully charged. The capacitor charging and dischargingoperations produce a transient voltage that appears at the output node310 of the PGA 300. If the transient voltage does not settle quicklyenough then it can cause false errors during the CRC calculations thatare performed by the DSP 112 during demodulation. Therefore, it ispreferable to minimize the effects of the transient voltages by settlingthe transient voltages as quickly as possible.

[0041] The settling time of the transient voltage can be reduced byclosing additional fine stage switches 306, beyond the particular finestage switch 306 that corresponds to the selected tap 304. By judiciallyclosing switches 306 in non-selected groups 312, the parasiticcapacitance for the fine stage switches 306 is pre-charged, therebyreducing the settling time of the transient voltage that accompanies achange in gain settings. The following sections describe two suchconfigurations that reduce the transient voltage settling time byclosing the additional fine stage switches 306 in non-selected groups312.

[0042] 5. Turn-on at Least One Switch in Each Group

[0043] FIGS. 4A-4B illustrate one embodiment for reducing transientvoltage settling time by closing additional switches 306 in non-selectedgroups 312. In this embodiment, at least one switch 306 is closed ineach group 312, even in those groups 312 that do not have the selectedtap 304. The switches 306 that are closed in the non-selected groups 312have the same corresponding location (or index) as for the selected tap304. The following examples further illustrate the switches 306 that areclosed in the non-selected groups 312.

[0044] For example, in FIG. 4A, the tap 304-11 is the selected tap inthe selected group 312-1, and therefore switches 306-11 and 308-1 areclosed. Additionally, the following fine stages switches 304 in thenon-selected groups 312 are also closed: switch 306-21 (in group 312-2),switch 306-31 (in group 312-3), and switch 306-n1 (in group 312 n), etc.Therefore, at least one switch 306 in each group 312 is closed at alltimes, which pre-charges the parasitic capacitance of the switches 306in each group 312 by some amount. By pre-charging the parasiticcapacitances, the transient voltage is reduced when the tap selection ischanged to a new group 312. The coarse stage switches 308-2, 308-3, and308-n for the corresponding non-selected groups 312-2, 312-3 and 312-nare left open, thereby isolating the corresponding fine stage switches306 in these groups from the output 310.

[0045] The closed switches 306 in the non-selected groups 308 have thesame location (or “index”) within the group 312 as for the selectedswitch 306-11 in the selected group 312-1. In other words, the selectedtap 304-11 is the first tap in the group 312, and the correspondingswitch 306 is the first switch in the group 312. Likewise, the closedswitches 306-21, 306-31, and 306-n1 are also the first switches in theirrespective groups 312.

[0046]FIG. 4B illustrates a second example for this embodiment, wherethe tap 304-22 is the selected tap in the selected group 312-2. Theswitches 306-22 and 308-2 are closed to connect the selected tap 304-22to the output 310.

[0047] Additionally, the following fine stage switches in thenon-selected groups 312 are also closed: switch 306-12 (in group 312-1),switch 306-32 (in group 312-3), and switch 306-n2 (in group 312-n), etc.The corresponding coarse stage switches 308-1, 308-3, and 308-n are leftopen.

[0048] 6. Turn-on Switches in Each Group in a Mirror Image Order

[0049] In a second embodiment, some of the closed switches 306 innon-selected groups 312 have a different relative location when comparedto the location of the selected tap 304. More specifically, the locationof the closed switches 306 in the non-selected groups is the mirrorimage of the location in an adjacent group 312. FIGS. 5A-5E furtherillustrate the location of the closed switches 306 in the non-selectedgroups 312 according to this mirror image embodiment. FIG. 5Aillustrates an initial switch configuration for an initial attenuationsetting. FIGS. 5B-5E illustrate the progression of switch configurationsfor increased attenuation and the switch operation in non-selectedgroups 312. As in prior sections, the switches 306 in the non-selectedgroups 312 are closed to pre-charge the parasitic capacitance that isassociated with the switches 306 and 308.

[0050] In FIGS. 5A-5E, it is noted that the number of switches 306 ineach group 312 is set to n=4 for ease of discussion. As will beapparent, each group 312 could contain any number of switches 306.Furthermore, in FIGS. 5A-5E, the fine stage switches 306 are arrangedinto five groups 312 (312-1 to 312-5). As will be apparent, the finestage switches 306 can be arranged into any number of groups.

[0051] In FIG. 5A, the tap 304-11 is the selected tap in the selectedgroup 312-1. The tap 304-11 is at the absolute top of the resistor 302so the signal attenuation to the output node 310 is a minimum. Theswitches 306-11 and 308-1 are closed to connect the selected tap 304-11to the output 310. Additionally, the switches 306-24, 306-31, 306-44,and 306-51 in the corresponding non-selected groups 312-2 to 312-5 arealso closed, so as to pre-charge the associated parasitic capacitance314 for the corresponding non-selected groups.

[0052] It is noted that the locations of the switches 306 that areclosed varies from over the groups 312. More specifically, the closedswitches 306 in adjacent groups 312 are at mirror image locations aboutthe boundary between the groups 312. For example, the selected switch306-11 in FIG. 5A is the first switch in the group 312- 1, and theswitch 306-24 is the last switch in the group 312-2, which is the mirrorimage of the switch 306-11 about a boundary 316-1 between the groups312-1 and 312-2. The switch 306-31 is the first switch in the group312-3, which is the mirror image of the switch 306-24 in group 312-2about a boundary 316-2 between the group 312-2 and 312-3. The switch306-44 is the last switch in the group 312-4, which is the mirror imageof the switch 306-31 in group 312-3 about a boundary 316-3 between thegroups 312-3 and 312-4. The switch 306-51 is the first switch in thegroup 312-5, which is the mirror image of the switch 306-44 in the group312-4 about a boundary 316-4 between the groups 312-4 and 312-5.

[0053] In FIG. SB, tap 304-12 is the selected tap, and therefore theswitches 306-12 and 308-1 are closed to connect the selected tap 304-12to the output 310. Additionally, the switches 306-23, 306-32, 306-43,and 306-52 are closed in the corresponding non-selected groups 312-2 to312-5, so as to pre-charge the parasitic capacitances of the switches306 in these non-selected groups.

[0054] As in FIG. 5A, the closed switches 306 in adjacent groups 312 areat mirror image locations about the boundary between the adjacent groups312. For example, the selected switch 306-12 is the second switch in thegroup 312-1, and the switch 306-23 is the third switch in the group312-2, which is the mirror image of the selected switch 306-12 about theboundary 316-1. The switch 306-32 is the second switch in the group312-3, which is the mirror image of the switch 306-23 in group 312-2about the boundary 316-2. The switch 306-43 is the third switch in thegroup 312-4, which is the mirror image of the switch 306-32 in group312-3 about the boundary 316-3. The switch 306-52 is the second switchin the group 312-5, which is the mirror image of the switch 306-43 inthe group 312-4 about the boundary 316-4.

[0055] In FIG. 5C, tap 304-13 is the selected tap, and therefore theswitches 306-13 and 308-1 are closed to connect the selected tap 304-13to the output 310. Additionally, the switches 306-22, 306-33, 306-42,and 306-53 in the corresponding non-selected groups 312-2 to 312-5 arealso closed, so as to pre-charge the parasitic capacitances of theswitches 306 in the non-selected groups 312-2 to 312-5.

[0056] As in FIGS. 5A-5B, the closed switches 306 in adjacent groups 312in FIG. 5C are at mirror image locations about the boundary between theadjacent groups 312. For example, the selected switch 306-13 is thethird switch in the group 312-1, and the switch 306-22 is the secondswitch in the group 312-2, which is the mirror image of the switch306-13 in group 312-1 about the boundary 316-1. The switch 306-33 is thethird switch in the group 312-3, which is the mirror image of the switch306-22 in the group 312-2 about the boundary 316-2. The switch 306-42 isthe second switch in the group 312-4, which is the mirror image of theswitch 306-33 in the group 312-3 about the boundary 316-3. The switch306-53 is the third switch in the group 312-5, which is the mirror imageof the switch 306-42 in the group 312-4 about the boundary 316-4.

[0057] In FIG. 5D, the tap 304-14 is the selected tap, and therefore theswitches 306-14 and 308-1 are closed to connect the selected tap 304-14to the output 310. Additionally, the switches 306-21, 306-34, 306-41,and 306-54 in the corresponding non-selected groups 312-2 to 312-5 arealso closed, so as to pre-charge the parasitic capacitances of theswitches 306 in the non-selected groups 312-2 to 312-5.

[0058] As in FIGS. 5A-5C, the closed switches 306 in adjacent groups 312are at mirror image locations about the boundary between the adjacentgroups 312. For example, the selected switch 306-14 is the last switchin the group 312-1, and the switch 306-21 is the first switch in thegroup 312-2, which is the mirror image of the switch 306-14 in group312-1 about the boundary 316-1. The switch 306-34 is the fourth switchin the group 312-3, which is the mirror image of the switch 306-21 ingroup 312-2 about the boundary 316-2. The switch 306-41 is the firstswitch in the group 312-4, which is the mirror image of the switch306-34 in the group 312-3 about the boundary 316-3. The switch 306-54 isthe last switch in the group 312-5, which is the mirror image of theswitch 306-41 in the group 312-4 about the boundary 316-4.

[0059] In FIG. 5E, tap 304-21 is the selected tap, and therefore theswitches 306-21 and 308-2 are closed to connect the selected tap 304-21to the output 310. It is noted that switch 306-21 is already closedbecause of the mirror image switch closing process for non-selectedgroups 312 that is illustrated by FIGS. 5A-5D. Since switch 306-21 isalready closed, the parasitic capacitance that is associated with theswitch 306-21 and the group 312-2 is already charged-up. Thissignificantly reduces the transient voltage that is normally associatedwith tap changes, and improves the settling time for any transientvoltage that remains. For example, in embodiments, the transient voltageis reduced from 100 mv to as low as 10 mV.

[0060] As stated above, the closed switches 306 in adjacent groups 312are at mirror image locations about the boundary between the adjacentgroups 312. The position of the closed switches 306 can be described inan equivalent but different manner. To preface this discussion, it isnoted that the groups 312 are indexed from 1-to-n (e.g. 312-1, 312-2,etc.) Hence, there are even numbered groups 312 (e.g. 312-2, 312-4) andodd numbered groups 312 (e.g. 312-1, 312-3, 312-5). For convenience, itis assume that the selected switch 306 is the m^(th) switch (out of n)in a selected group 312. If the selected switch 306 is located in aneven numbered group 312 (e.g. 312-2, 312-4, etc.), then the m^(th)switch is closed in all the even numbered groups 312. Additionally, the[(n+1)−m^(th)] switch 306 is closed in all the odd numbered groups 312.Similarly, if the selected switch 306 is located in an odd numberedgroup 312 (e.g. 312-1, 312-3, etc.), then the m^(th) switch 306 isclosed in all the odd numbered groups 312, and the [(n+1)−m^(th)] isclosed in the even numbered groups 312.

[0061] As an example, in FIG. 5A, the tap 304-11 is the selected tap sothat the switches 306-11 and 308-1 are closed to connect the tap 304-11to the output 310. The switch 306-11 is the first switch in the group312-1, which is an odd numbered group. In accordance with the discussionabove, the first switches 306 in the odd numbered groups 312 are to beclosed. This is born out in FIG. 5A as switches 306-31 and 306-51 areclosed in the odd numbered groups 312-3 and 312-5, respectively.Additionally, the (n+1)−m^(th) switches are to be closed in the evennumbered groups according to the discussion above. Since n=4 (as thereare 4 switches in each group 312) and m=1 (as the first switch 306-11corresponds to the selected tap 304-11) , then:

(n+1)−m=(4+1)−1=4

[0062] Therefore, the 4th switch in the even numbered groups 312 is tobe closed. This is born out in FIG. 5A as switches 306-24 and 306-44 areclosed the groups 312-2 and 312-4, respectively. Note that switches306-24 and 306-44 are the fourth switches in FIGS. 5A-5E.

[0063] As a second example, in FIG. 5B, the tap 304-12 is the selectedtap so that the switches 306-12 and 308-1 are closed to connect tap304-12 to the output 310. The switch 306-12 is the second switch in thegroup 312-1, which is an odd numbered group. In accordance with thediscussion above, the second switch 306 in each odd numbered group 312is to be closed. This is born out in FIG. 5B as switches 306-32 and306-52 are closed in the odd numbered groups 312-3 and 312-5,respectively. Additionally, the [(n+1)−m]^(th) switch is to be closed ineach of the even numbered groups. Since n=4 and m=2, then:

(n+1)−m=(4+1)−2=3

[0064] Therefore, the 3rd switch in the even numbered groups 312 is tobe closed. This is born out in FIG. 5B as switches 306-23 and 306-43 areclosed the groups 312-2 and 312-4, respectively.

[0065] The operation of the PGA 300 is further described according toflowchart 700 that is shown in FIG. 7, which is described as follows.

[0066] In step 702, a gain control signal is received that determinesthe attenuation of the variable resistor 301, and therefore the gain ofthe PGA 300. The gain control signal identifies the selected tap 304that is to be connected to the output 310. For example, the gain controlsignal can be an automatic gain control (AGC) signal, such as AGC signal117 (FIG. 1) that is generated by the AGC module 116.

[0067] In step 704, the fine stage switch 306 and the coarse stageswitch 308 that correspond to the selected tap 304 are closed. The finestage switch 306 that corresponds to the selected tap 304 is identifiedas the m^(th) switch 306 (out of n) in the selected group 312. Forexample, in FIG. SA, tap 304-11 is the selected tap so that the finestage switch 306-11 and the coarse stage switch 308-1 are closed toconnect the selected tap 304-11 to the output 310. The switch 306-11 isthe first switch (out of 4) in the selected group 312-1.

[0068] In step 706, the determination is made as to whether the selectedtap 304 and corresponding switch 306 are in an even numbered group 312or an odd numbered group 312. If the selected tap 304 is in an evennumbered group 312, then control flows to step 708. If the selected tap304 is in an odd numbered group 312, then control flows to step 712. Forexample, in FIG. 5A, the selected tap 304-1 is in group 312-1, which isan odd numbered group.

[0069] In step 708, the selected tap 304 is in an even numbered group,therefore the m^(th) switch 306 is closed in each even numbered group312 that is a non-selected group 312 (Note that the switch correspondingto the selected tap 304 was closed in step 704). Additionally, in step710, the [(n+1)−m^(th)] switch 306 is closed in every odd numbered group312.

[0070] In step 712, the selected tap 304 is in an odd numbered group,therefore the m^(th) switch 306 is closed in every odd numbered group312 that is a non-selected group 312 (Note that the switch 306corresponding to the selected tap 304 was closed in step 704).Additionally, in step 714, the [(n+1)−m^(th)] switch 306 is closed inevery even numbered group 312. For example, in FIG. 5A, switches 306-31and 306-51 are closed in additional to switch 306-11.

[0071] In step 716, the flowchart ends.

[0072] 7. Transmission Line Characteristics of 2-Stage SwitchConfiguration

[0073] A further benefit of the PGA 300 with the 2-stage switchconfiguration is that the overall input impedance of the variableresistor 301 is closer to that of a transmission line. Referring to FIG.3B, the resistor 302 and the parallel effective capacitors 314 have adistributed characteristic that closely approximates the impedance of atransmission line, for example a cable. As a result, the 3 dB cutofffrequency substantially matches that of transmission line, asillustrated by curve 602 in FIG. 6.

[0074] The input impedance of PGA 300 appears as a distributed RCnetwork because the resistance and capacitance of the PGA 300 aredistributed through the two stages. As a result, the PGA 300 has anamplitude roll-off that varies as 1/{square root}{square root over(freq)}. Furthermore, in one embodiment, there is an inverserelationship between the PGA tap selection and the cable length (i.e.cable 102). For example, given a relatively short cable, tap 304-nn(FIG. 3A) can be selected to set a relatively high attenuation for thePGA 300. Given a relatively long cable, the tap 304-11 can be selectedto set a relatively low attenuation for the PGA 300. By using thisinverse relationship, less equalization is needed for the DSP 112.

[0075] 8. Multi-stage configurations

[0076] As described herein, the PGA 300 is a two-stage PGA. However, theinvention is not limited to a two-stage PGA, as the present inventioncan be implemented in a multistage PGA having more than two stages. Inother words, the switching configurations and methods described herein,can be implemented in a multi-stage PGA, as will be understood by thoseskilled in the arts based on the teachings given herein.

[0077] 9. Other Applications

[0078] The PGA invention described herein has been discussed inreference to a receiver. However, the PGA is not limited to receivers,and is applicable to other non-receiver applications that benefit fromlow transient voltages and good frequency bandwidth. The application ofthe PGA invention to these non-receiver applications will be understoodby those skilled in the relevant arts based on the discussions givenherein, and are within the scope and spirit of the present invention.

[0079] 10. Conclusion

[0080] Example embodiments of the methods, systems, and components ofthe present invention have been described herein. As noted elsewhere,these example embodiments have been described for illustrative purposesonly, and are not limiting. Other embodiments are possible and arecovered by the invention. Such other embodiments will be apparent topersons skilled in the relevant art(s) based on the teachings containedherein. Thus, the breadth and scope of the present invention should notbe limited by any of the above-described exemplary embodiments, butshould be defined only in accordance with the following claims and theirequivalents.

What is claimed is:
 1. A programmable gain amplifier (PGA), comprising:a resistor having a plurality of taps; a first group of n-fine stageswitches, each fine stage switch in said first group having an outputcoupled to a first group output terminal; a second group of n-fine stageswitches, each fine stage switch in said second group having an outputcoupled to a second group output terminal; each fine stage switch insaid first group and said second group having an input coupled to acorresponding tap on said resistor; a first coarse stage switch havingan input coupled to said first group output terminal, and an outputcoupled to an output of said PGA; a second coarse stage switch having aninput coupled to said second group output terminal, and an outputcoupled to said output of said PGA; and wherein if a m^(th) fine stageswitch is closed in said first group, then a ((n+1)−m^(th)) fine stageswitch is closed in said second group.
 2. The PGA of claim 1, furthercomprising an amplifier having an output coupled to an input of saidresistor.
 3. The PGA of claim 1, wherein said resistor is connected to areference voltage.
 4. The PGA of claim 1, wherein a location of said((n+1)−m^(th)) fine stage switch in said second group is a mirror imageof a location of said m^(th) fine stage in said first group.
 5. Aprogrammable gain amplifier (PGA), comprising: a resistor; a pluralityof fine stage switches having inputs coupled to corresponding taps onsaid resistor, said fine stage switches arranged into two or moregroups, each group having an output terminal and n-fine stage switchesthat are capable of being indexed from 1-to-n, said n-fine stageswitches having outputs coupled said output terminal for saidcorresponding group; and two or more coarse stage switches correspondingto said two or more groups, each coarse stage switch having an inputcoupled to said output terminal for said corresponding group, eachcoarse stage switch having an output coupled an output of the PGA;wherein if a m^(th) fine stage switch is closed in a first group, then a((n+1)−m)^(th) fine stage switch is closed in a second group that isadjacent to said first group.
 6. A programmable gain amplifier (PGA),comprising: an amplifier; a resistor having a first terminal coupled toan output of said amplifier and a second terminal coupled to a referencevoltage, said resistor having a plurality of taps between said firstterminal and said second terminal; a plurality of fine stage switcheshaving inputs coupled to corresponding taps on said resistor, said finestage switches arranged into two or more groups, each group having anoutput terminal and n-fine stage switches that are capable of beingindexed from 1-to-n, each n-fine stage switch in said groups having anoutput coupled said output terminal for said corresponding group; andtwo or more coarse stage switches corresponding to said two or moregroups, each coarse stage switch having an input coupled to said outputterminal for said corresponding group, each coarse stage switch havingan output coupled an output of the PGA; wherein if a m^(th) fine stageswitch is closed in a first group, then a ((n+1)−m)^(th) fine stageswitch is closed in a second group that is adjacent to said first group.7. The PGA of claim 6, wherein said resistor, said plurality of finestage switches, and said two or more coarse stage switches are depositedon a common substrate.
 8. The PGA of claim 7, wherein said amplifier isalso deposited on said common substrate.
 9. The PGA of claim 7, whereinsaid common substrate is CMOS.
 10. The PGA of claim 6, wherein saidamplifier, said resistor, said plurality of fine stage switches, andsaid two or more coarse stage switches are deposited on a common CMOSsubstrate.
 11. A method of adjusting the gain of a programmable gainamplifier (PGA), the PGA having a resistor with a plurality of taps anda switch network between the taps and an output of said PGA, the switchnetwork having a plurality of fine stage switches coupled to saidplurality of taps, the taps and corresponding fine stage switchesarranged into two or more groups, the two or more groups including evennumbered groups and odd numbered groups, the method comprising the stepsof: receiving a gain control signal that identifies a selected tap ofsaid plurality of taps that is to be connected to the PGA output; ifsaid selected tap is in an even numbered group, then closing a m^(th)fine stage switch in each even-numbered group, and closing a[(n+1)−m]^(th) fine stage switch in each odd-numbered group.
 12. Themethod of claim 11, further comprising the step of: if said selected tapis in an odd numbered group, then closing a m^(th) fine stage switch ineach odd-numbered group, and closing a [(n+1)−m]^(th) fine stage switchin each even-numbered group.
 13. The method of claim 11, furthercomprising the step of closing a coarse stage switch that corresponds toa group having said selected tap.
 14. The method of claim 13, furthercomprising the step opening said coarse stage switch and closing asecond coarse stage switch to adjust the gain of said PGA, withoutopening a previously closed fine stage switch.
 15. A method of adjustingthe gain of a programmable gain amplifier (PGA), the PGA having aresistor with a plurality of taps and a switch network between the tapsand an output of said PGA, the switch network having a plurality of finestage switches coupled to said plurality of taps, the taps andcorresponding fine stage switches arranged into two or more groups, thetwo or more groups including even numbered groups and odd numberedgroups, the method comprising the steps of: closing a m^(th) fine stageswitch that corresponds to a selected tap of said plurality of taps in aselected group of said plurality of groups; if said selected tap is oneof said even numbered groups, then closing a m^(th) fine stage switch ineach of said even-numbered groups that is a non-selected group, andclosing a [(n+1)−m]^(th) fine stage switch in each one of saidodd-numbered groups; and if said selected tap is one of said oddnumbered groups, then closing a m^(th) fine stage switch in each of saidodd-numbered groups that is a non-selected group, and closing a[(n+1)−m]^(th) fine stage switch in each one of said even-numberedgroups.
 16. The method of claim 15, further comprising the step ofclosing a coarse stage switch that corresponds to said selected group.17. The method of claim 16, further comprising the step opening saidcoarse stage switch and closing a second coarse stage switch to adjustthe gain of said PGA, without opening said m^(th) fine stage switch thatcorresponds to said selected tap.
 18. A method of adjusting the gain ofa programmable gain amplifier (PGA), the PGA having a resistor with aplurality of taps and a switch network between the taps and an output ofsaid PGA, the switch network having a plurality of fine stage switchescoupled to said plurality of taps, the fine stage switches arranged intotwo or more groups, the method comprising the steps of: closing a firstfine stage switch in a first group of said two or more groups; andclosing a second fine stage switch in a second group of said two or moregroups, said second group adjacent to said first group and defining aboundary between said first group and said second group, said secondfine stage switch located at a mirror image about said boundary relativeto a location of said first fine stage switch.
 19. The method of claim18, further comprising the step of closing a coarse stage switch thatcorresponds to said first group.
 20. The method of claim 19, furthercomprising the steps of opening said coarse stage switch thatcorresponds to said first group, and closing a second coarse stageswitch that corresponds to said second group.